The divergence problem has been puzzling dendrochronologists (tree ring scientists) for several decades, and although you may not have heard about it, it is really important. Tree rings are strongly influenced by climate and this has made them very useful as a record of Earth's climate, with some chronologies spanning many thousands of years (built using samples from living trees, building timbers and sub-fossils from bogs or lakes).

While early researchers used the width of rings as a record of climate, they later found that the density of the wood was an even better record of past climate. However, there was a problem - the divergence problem.

Over the last 50 years, climate has shown a well-known warming trend, but the wood density of tree rings has not tracked this trend. Several explanations have been proposed, but it has proved tricky to find supporting evidence.

In a new study Alexander Stine and others report evidence in support of the role of "global dimming", a reduction in solar radiation reaching the Earth's surface that has been observed since the 1960's. In brighter parts of the Arctic they show that the divergence problem is largely absent in their tree rings, while in the parts of the Arctic that have "dimmed" the most, the divergence is also greatest.